This invention relates generally to pollution control and more particularly to methods and apparatus for reducing and recovering dust emissions from air conveyors.
Air conveyors are used for moving very dry, fluid materials, such as powdered substances at cement plants, lime plants, pharmaceutical plants, paint plants, and other facilities. The powdered material being moved is typically very fluid, and resembles face powder in that regard. At least one known fluidizing air conveyor has two chambers joined at a horizontal flange. In fact, the flange is not actually horizontal, but rather is inclined slightly (for example, at 7 to 8 degrees) to allow the conveyor to advantageously utilize the force of gravity to move the fluidized material from a silo or other material reservoir. An air-permeable fabric or felt is sandwiched in the flange. Below the flange, one of the chambers is an open metal box with pressurized air covered by the fabric. Because of the pressure differential between the lower chamber and the upper chamber (in some typical instances, for example, between 3 to 5 psi or 20.67 to 34.45 kpascal), air “bubbles up” through the fabric. Powder is dropped from a reservoir such as a silo into an opening in the elevated end of top chamber 18. That powder is transferred some distance (e.g., 50 to 100 feet in some cases) to where it is used or otherwise dispensed. By fluidizing the material with the pressurized air bubbled through the fabric and through the product, the material flows more easily. The slight incline allows gravity to cause the fluidized material to flow down the conveyor. The chamber below the fabric or felt is pressurized and the air supports and fluidizes the material so that it flows down, assisted by the air.
In some known configurations, a bottom chamber of the air conveyor is 2-4″ (5.08-10.16 cm) tall and up to 24″ (60.96 cm) wide, and a top chamber is 4-12″ (10.16-30.48 cm) tall and up to 24″ (60.96 cm) wide, depending upon how much material is being moved. The powdered material flows over the fabric that is laid as a top cover over the lower chamber, and flows down what is effectively a long, rectangular tube. Various types of fabrics are used in known configurations, depending upon the type of material being conveyed. For example, for very hot material, NOMEX® fabric (available from DuPont Corporation) can be used. For materials at room temperature, a polyester, felt polyester or other woven material can be used.
In known configurations, when the pressurized air exits at the discharge area for the conveyed material, the escaping air has a tendency to carry material dust into the atmosphere. Although a baghouse is sometimes used to prevent the material dust from entering the atmosphere, many material conveyors are in areas that are not generally accessible with low headroom clearances. In such installations, there is no room for a baghouse. In other cases, a baghouse is not practical to provide or is too expensive, because there also must be some way of collecting dust in the baghouse and feeding it back into the air conveyor, and a fan is required.